Number | Name | Date | Kind |
---|---|---|---|
2279553 | Bradt | Apr 1942 | |
3062110 | Shepardson et al. | Nov 1962 | |
3570224 | Clemens | Mar 1971 | |
3680528 | Sanders | Aug 1972 | |
3894514 | Caudill et al. | Jul 1975 | |
4399209 | Sanders et al. | Aug 1983 | |
4440846 | Sanders et al. | Apr 1984 | |
4678489 | Bertelsen | Jul 1987 | |
4878581 | Beery | Nov 1989 |
Entry |
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Research Disclosure No. 301118 entitled, "A Method and System of Assuring Filter Changes in an Unmonitored Enrivonment", dated May 1989. |